1. Field of the Invention
The invention is concerned with an electrode arrangement suitable for bloodless measurements in connection with the concentration or the partial pressure of a gas in the blood.
2. Description of the Prior Art
It is known to bloodlessly measure the partial pressure of a gas in the blood (e.g. of oxygen or CO.sub.2) with electrodes. Generally, an electrode is applied to a position of the body surface of the patient, for example the scalp, a wrist, the inner canthus of an eye, etc. With the presence of an initial potential at the electrode, an electrochemical reaction takes place at its surface, whereby gas is diffused through the tissue from the blood (e.g. in the case of oxygen, its reduction to OH.sup.- ions). The resulting current is measurable and under certain circumstances is proportional to the concentration or the partial pressure of the gas in the blood.
Bare electrodes or electrodes covered with a membrane are suitable, depending on the particular measurement requirements, and platinum is preferred for the electrode material. Typical materials for the membrane material would include polypropylene, polyethylene, teflon, mylar, etc.
It has been found that the measured current values derived by the electrodes are not only a function of the actual partial pressure of a gas in the blood, but, in addition, depend on the local blood circulation of the tissue in the electrode measuring region. Because of the latter, for some measurements, it would be advantageous to decrease the influence of the blood circulation on the measuring results by hyperaemising the skin in the measuring region to increase the blood volume. It has already been proposed to use suitable vasodilating preparations (for example, histamine, papaverine, nicotinic acid, etc.) for this purpose. However, the application of such preparations is problematical in certain cases, for example, in the case of measurement on the inner canthus of the eye.